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In the class E high-efficiency switching-mode tuned power amplifier, the RF output amplitude is almost exactly a linear function of the collector dc supply voltage, since conduction angles and wave shapes are not changed by the dc supply voltage. Thus the RF output can be amplitude-modulated by varying the dc supply voltage in accordance with desired amplitude-modulation (AM) envelope. Deviations from the ideal linear modulation characteristic are caused by: transistor saturation voltage; RF feedthrough from the driver via C₂₁ ; nonunity transmission of the ac modulating signal into the amplifier through the nonzero inductance of the collector shuntfeed RF choke; and differences in the magnitudes and delay times, of the RF transmission function through the RF output network, of three different frequencies: lower sideband, carrier, and upper sideband. Design criteria are given, to hold the modulation distortion below a limit chosen by the designer. Experimental results agree well with analytical predictions. Distortion of the modulation envelope can easily be held below 1 percent.
Marian K. Kazimierczuk (Fri,) studied this question.